Circuit for uniformly anodizing thin-film resistors

ABSTRACT

A THIN-FILM RESISTOR IS ANODIZED TO INCREASE ITS RESISTANCE TO A PRESELECTED VALUE WHILE CONNECTED AS ONE ARM OF A BRIDGE CIRCUIT WHICH UTILIZES AN A.C. VOLTAGE FOR A RESISTANCE MEASUREMENT. OPPOSITE ENDS OF THE PRIMARY WINDING OF A TRANSFORMER ARE CONNECTED ACROSS THE MEASURING DIAGONAL OF THE BRIDGE. A D.C. ANODIZING CURRENT SOURCE IS CONNECTED TO A CENTER TAP OF THE PRIMARY TRANSFORMER WINDING AND TO THE THIN-FILM RESISTOR THROUGH AN ELECTROLYTE.   THE RESISTANCE VALUES OF THE BRIDGE ARE SUCH THAT THE ANODIZING CURRENT FLOWING TO THE CENTER TAP OF THE TRANSFORMER WINDING DIVIDES AND FLOWS IN EQUAL AMOUNTS IN OPPOSITE DIRECTIONS THROUGH THE WINDING. THE ANODIZATION CURRENT, THEREFORE, FLOWS UNIFORMLY THROUGH ALL PARTS OF THE THIN-FILM RESISTOR TO ANODIZE THE RESISTOR UNIFORMLY AND MAINTAIN ITS LINERAITY.

Feb. 9, 1971 R W RD I v 3,562,133

CIRCUIT FOR UNIFORMLY ANODIZING THIN-FILM RESISTORS Filed Nov. 18. 1968 I l Z7\/\/E7/\/7"UR L w i J RE. U/FIE'D United States Patent M 3,562,133 CIRCUIT FOR UNIFORMLY ANODIZING THIN-FILM RESISTORS Robert C. Ward, Winston-Salem, N.C., assignor to Western Electric Company, Incorporated, New York, N.Y.,

a corporation of New York Filed Nov. 18, 1968, Ser. No. 776,396 Int. Cl. B01k 3/00 U.S. Cl. 204-228 9 Claims ABSTRACT OF THE DISCLOSURE A thin-film resistor is anodized to increase its resistance to a preselected value while connected as one arm of a bridge circuit which utilizes an A.C. voltage for a resistance measurement. Opposite ends of the primary winding of a transformer are connected across the measuring diagonal of the bridge. A DC. anodizing current source is connected to a center tap of the primary transformer winding and to the thin-film resistor through an electrolyte. The resistance values of the bridge are such that the anodizing current flowing to the center tap of the transformer winding divides and flows in equal amounts in opposite directions through the winding. The anodization current, therefore, flows uniformly through all parts of the thin-film resistor to anodize the resistor uniformly and maintain its linearity.

CROSS REFERENCE TO RELATED APPLICATION This application is related to an application filed in the names of T. L. Crisman and R. C. Ward on even date herewith.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention relates to a control circuit for anodizing thin-film resistors. A thin-film resistor may include a layer of resistive metal deposited, by a technique such as sputtering, in a linear pattern upon an insulative substrate. Since it is very diflicult to deposit the exact thickness of metal necessary to produce a preselected value of resistance, the usual technique is to initially deposit a thickness of metal greater than desired in order to produce a resistance lower than the preselected value. The resistive film is then anodized to remove some of the metal by oxidation and reduce the cross-sectional area of the film and thereby increase its resistivity. Anodization is continued until the resistance of the thin-film resistor reaches the desired, preselected value.

One problem encountered in anodizing resistors is that precise resistor values can not be attained if the resistance is not monitored during the anodization process. Further, in anodizing resistors for certain uses, it is desirable that the resistance of the finished resistor be linear as a function of distance along the resistor.

(2) Description of the prior art In the past, a number of diiferent circuits have been developed in which the resistance of a resistor being anodized is monitored by a bridge circuit to stop the flow of anodizing current when the bridge is balanced. In certain types of these prior circuits, the resistor is alternately connected to the monitoring and anodizing circuits, so that part of the time, the resistance is being measured and the remaining time the resistor is being anodized. With the method of alternate anodization and monitoring, the resistor is only anodized during part of the time so that the total process of anodization takes longer. Further, the

danger exists that the resistance of the resistor being anodized may pass through the desired value during the last cycle of anodization and be outside the desired tolerance range. If the resistance of the resistor being anodized is continuously monitored during the anodization process such as with a Wheatstone bridge, the anodization current flowing in the resistor disturbs the operation of the bridge and results in resistance measurement errors.

One technique which has been employed to overcome the errors characteristic of continuous anodization and measurement is that of utilizing an A.C. current to measure and a DC. current to anodize. Prior art circuits utilizing this technique, however, do not have equal amounts of anodizing current flowing through every portion of the resistor to be anodized, hence, the resistance of the resistive film is not uniform and is unsuitable for certain uses. Also, these prior art circuits do not eliminate all error in the resistance measurement due to the anodizing current.

SUMMARY OF THE INVENTION The object of the invention is a new and improved anodizing control circuit and method. In one embodiment of the circuit, a resistor to be anodized is connected to one terminal of an anodization current supply source through an electrolyte. Opposite ends of the resistor to be anodized are separately connected to the other terminal of the anodization current supply through equal values of resistance. Because of this balanced connection, equal anodization currents flow from each end of the resistor to the anodization current supply terminal and the resistor is uniformly anodized. More particularly, the circuit additionally incorporates a transformer connected across the output terminals of a bridge circuit for monitoring the value of the resistor being anodized. Anodizing currents is applied through a center tap on the primary of the transformer to the opposite ends of the resistor, thus eliminating the effect of the anodizing current on the measurement of the resistor.

BRIEF DESCRIPTION OF THE DRAWING The nature of the present invention and its various advantages will appear more fully by referring to the following detailed description in conjunction with the appended drawing, in which:

FIG. 1 is a schematic drawing of an anodizing control circuit utilizing A.C. measurement and DC anodization constructed in accordance with the invention;

FIG. 2 is a simplified schematic drawing of a portion of FIG. 1 illustrating the current flow within the circuit; and

FIG. 3 is a schematic drawing of an alternate embodiment of a bridge balance detector which may be used with the invention.

DETAILED DESCRIPTION Referring to FIG. 1, a thin-film resistor R which is to be anodized may comprise a resistive pattern of metallic film deposited upon an insulative substrate, such as tantalum nitride, upon a ceramic. The resistance of the resistor Rtf is increased to a preselected value by anodizing the resistive film to reduce the cross-section of conductive material and thereby increase its resistivity. An anodizing current is applied to the resistor from an electrode 10 through an electrolyte 11 which contacts the resistive film. As the resistive film is anodized, a bridge circuit 12 produces an output signal which is indicative of the variation of the resistance value of the resistive film from a preselected standard value of resistance R When the resistive film reaches the preselected value, a monitor circuit 13 connected to the output of the bridge 3 circuit 12 operates to interrupt the flow of anodizing current.

With the thin-film resistor R connected as the first arm of the bridge circuit 12, the circuit also includes a second resistive arm R which may be adjusted to a preselected standard value, and third and fourth resistive arms R and R respectively. A pair of resistors R and R are connected in series between the respective ends of the adjustable resistor R and the thin-film resistor R The resistance of the parallel combination of the resistors R and R is chosen to be equal to the parallel combination of the resistors R and R to insure a balanced flow of anodization current through the thin-film resistor R as will be discussed below. For convenience, the resistance values may be chosen so that R =R :R =R In the bridge circuit 12, the resistors R and R are shown in a conventional Kelvin double bridge arrangement wherein the resistors in the positions of R and R serve to divide the contact resistance and lead resistance error on one side of the bridge similar to the manner in which the resistors R and R divide these resistances. For certain purposes, contact resistance, even though in the range of a fraction of an ohm, may become significant.

In place of the usual galvanometer, the primary winding 19 of a first transformer T is connected in series across the measuring diagonal of the bridge 12 between the junction of the resistors R and R and the junction of the resistors R and R The secondary winding 21 of the transformer T is connected to the input of a phase detector 22 whose output is connected to a balance indicating voltmeter 23. The transformer T may include windings 19 and 21 whose ratio is 600 ohms to 50K ohms.

The A.C. power supply includes an alternating current generator 24 connected to the primary winding 25 of a second transformer T The secondary winding 26 of the transformer T is connected across one diagonal of the bridge circuit 12. Another secondary or tertiary winding 27 of the transformer 23 is connected to a second input of the phase detector 22. The transformer T may comprise a standard filament transformer so that the ratios of voltages on the windings 25, 26, 27 are 115:6:6.

The transformer windings are initially connected to the phase detector 22 so that the phase difference in the voltages across opposite diagonals of the bridge 12 is zero. As the bridge 12 passes through a balanced condition, the current flowing through the windings of transformer T go to zero and if the resistance of the resistor R would continue to be increased, current would flow in the opposite direction through the windings of T indicating a 180 phase reversal. The phase detector 22 is of the type that produces no output voltage when the two input voltages are in phase and generates a step-function of predetermined voltages when the two input voltages reverse in phase by 180". The production of an output voltage by the phase detector 22 is therefore indicative of the fact that the thin-film resistor R is equal in value to the standard resistor R and that the bridge 12 is balanced. The output voltage is used to actuate an anodization current interrupt relay 31 and the presence of the voltage is indicated to the operator by a voltmeter 23.

It is also to be understood that it is not essential that a phase-sensitive detection means 13 be used. As shown in FIG. 3, a voltage null detector could be used in the place of the phase-sensitive detector with equal effectiveness. When the bridge 11 is unbalanced, a voltage will be produced across the secondary winding 21 of the transformer T which will result in an output from the amplifier 40. As long as the amplifier delivers a voltage to the input of a gate 41, the gate output is inhibited and the relay 31 remains unoperated. However, when the bridge 11 is balanced, the output of the amplifier 40 is zero and the gate 41 produces an output voltage to operate the relay 31 and interrupt the flow of anodization current through the contacts 29.

The positive terminal of a D.C. anodizing current source 28 is connected to a center tap of the primary winding 19 of the transformer T The negative terminal of the voltage source 28 is connected through the contacts 20 of the anodization current interrupt relay 31 to a cathode electrode 10. The cathode electrode 10 is in electrical anodizing contact with the thin-film resistor K through a body of electrolyte 11 which may comprise a solution of 0.01% citric acid. The anodization current source 28 may be arranged to deliver values of anodization current ranging from 10 microamps to 1 milliamp depending upon the surface area of the resistive material to be anodized.

In operation, an A.C. measurement voltage is applied from the power supply 15 to the diagonals of the bridge 12. The frequency of the measurement voltage may be chosen as Hz. so that interference from 60 Hz. line current and its harmonics are avoided; however, the interference has been found to be substantially inconsequential and a 60 Hz. measurement voltage performs satisfactorily. The standard resistor R is set to be equal to the final, preselected value of resistance to which the thin-film resistor 10 is to be anodized. Since each of the resistors R R R and R are equal (2K ohms when R is in the range of 4K ohms) and the thin-film resistor R is initially a very low value, the bridge 12 will be unbalanced and no phase difference will exist between the current flowing through the secondary winding 21 of the transformer T and the tertiary winding 27 of the transformer T The phase detector 22 does not produce an output voltage as long as there is no phase difference between the two input voltages; therefore, the relay 31 remains unactuated so that the contacts 29 are closed and anodization current is delivered to the cathode electrode 10.

The thin-film resistor R continues to be anodized and its value is increased toward that preselected for the standard resistor R When the value of the thin-film resistor R is increased to become equal to the value of the standard resistor R the bridge 12 becomes balanced and the phase difference between the two input voltages to the phase detector 22 reverses by The phase detector 22 produces an output voltage which energizes the windings of the relay 31 and opens the contacts 29 to interrupt the flow of anodization current to the cathode. The output voltage from the phase detector 22 is also registered on a voltmeter 23 to indicate to the operator that the anodization process is finished and the thin-film resistor R should be removed.

A particular feature of the invention resides in the uniformity with which a thin-film resistor may be anodized and, for some applications, very precise linearity in the resistance of a resistor is critical. The linearity of a thin-film resistor may be ascertained by inspecting the color of the resistor. That is, a tantalum thin-film resistor is initially a uniform dark, almost black, color. However, as anodization takes place and the thickness of the metal film is decreased, the color of the film of oxidized material changes and takes on various hues. The uniformity of the hue is an indication of the uniformity of the thickness of the tantalum pentoxide film. A resistor which is a darker shade at one end and progresses to a lighter shade at the other end is of a non-uniform thickness of tantalum film and therefore of a non-uniform resistivity over the length of the film.

The circuit employed in the present invention yields an anodized resistor having a high degree of uniformity in the thickness and resistivity of the tantalum film.

The manner in which this is accomplished may be illustrated by referring to FIG. 2. Since the DC. path through which the anodization current flows includes the secondary windings 26 of the transformer T which is of very low resistance, the two resistors R and R may be effectively combined in parallel to form a composite resistance R having a value equal to one-half that of either of the two resistors. Likewise, resistors R and R may be combined into a composite resistor R Since the center tap of the primary winding 19 of the transformer T is connected to the positive terminal of the anodization voltage source 28, the anodization current flowing through the thin-film resistor sees a path of equal resistance through each half of the primary winding 19. That is, the anodization current I tends to split into two equal halves and to flow in opposite directions around the circuit, through the respective resistors R and R into opposite ends of the thin-film resistor R In this manner, equal amounts of current flow from the cathode electrode to opposite ends of the resistor R Equal amounts of current flow through every portion of the resistive film of the thin-film resistor R4 and this uniform current distribution in the resistor assures a uniform anodization of the film.

The division of the anodization current into two halves flowing in opposite directions through the primary winding 19 of the transformer T also serves to cancel out any contributions of anodizing current which might otherwise be sensed as a portion of a measuring current. Further, this equal and opposite flow through the transformer winding creates a balanced flux in opposite directions to prevent saturation of the transformer core due to anodization current.

The use of an A.C. voltage to measure the resistance of the thin-film resistor R as itis being anodized, avoids the problem of detecting a portion of the DC. anodization current as measurement current and thereby obtaining an erroneous resistance measurement. Secondly, the use of an A.C. current for measurement contributes very little to the anodization of the thin-film resistor 10 to impair its linearity, because the current cancels out over opposite halves of each cycle.

It is to be understood that the above-described embodiments are simply illustrative of the invention and that many other embodiments can be devised without departing from the scope and spirit of the invention.

What is claimed is:

1. A circuit for uniformly anodizing a thin-film resistor to increase the resistance of said resistor, comprising:

a source of anodizing current including a voltage source having a first and a second output terminal;

means for connecting the first output terminal of said voltage source to said thin-film resistor through an anodizing electrolyte;

first resistance means for connecting one end of said thin-film resistor to the second output terminal of said voltage source;

second resistive means for connecting the other end of said thin-film resistor to the second output terminal of said voltage source, the resistance of said second resistive means being equal to that of said first resistance means so that the currents flowing between the first output terminal of said voltage source and opposite ends of said thin-film resistor are equal and said resistor is uniformly anodized thereby.

2. A circuit for uniformly anodizing a thin-film resistor to increase the resistance of said resistor, comprising:

a voltage source having a pair of output terminals;

means for connecting a first of the output terminals of said voltage source to said thin-film resistor through an anodizing electrolyte;

a pair of equal, resistive paths respectively connecting opposite ends of said thin-film resistor to the second output terminal of said voltage source so that equal currents flow between said first output terminal and opposite ends of said thin-film resistor resulting in uniform anodization of all areas of said film.

3. A circuit for controlling the anodization of a thinfilm resistor to increase the resistance of said resistor to a preselected value while the thickness of said film remains uniform, comprising:

means for monitoring the resistance of said thin-film resistor and producing an output signal in response to the resistance of said resistor being equal to said preselected value;

a source of anodizing current having first and second terminals;

means for connecting the first terminal of said anodizing current source to said thin-film resistor through an anodizing electrolyte;

circuit means for connecting each end of said thin-film resistor of the second terminal of said anodizing current source, the resistance from one end of said thinfilm resistor to the second terminal being equal to the resistance from the other end of said thin-film to the second terminal to insure that an equal amount of anodizing current flows in all portions of said thinfilm resistor to uniformly decrease the thickness of the thin-film of said resistor; and

means responsive to said output signal for interrupting the flow of anodizing current.

4. A circuit for anodizing a thin-film resistor to increase the resistance of the resistor, comprising:

a resistive measuring bridge circuit having a pair of input terminals and a pair of output terminals and wherein the thin-film resistor is connected into one arm of the bridge circuit;

means for connecting an A.C. voltage to the pair of input terminals;

a transformer having a primary winding connected across the pair of output terminals;

means connected to a secondary winding of the transformer to sense the balance condition of the bridge circuit; and

means for connecting a DC. anodizing voltage across an anodizing electrolyte contacting the thin-film resistor and a center tap on the primary winding of the transformer.

5. A circuit for controlling the anodization of a thinfilm resistor to increase the resistance of said resistor to a preselected value, while maintaining the linearity of said resistor, comprising:

a bridge circuit having said thin-film resistor connected as a first arm, a resistor equal to said preselected value connected as a second arm, and a pair of equal resistors connected, respectively, as third and fourth arms of said bridge; secondary winding of a first transformer connected across the first and second arms of said bridge, the primary winding of said first transformer being connected to a source of A.C. voltage to provide a standard reference voltage for measuring the resistance of said first arm;

a resistor having a value equal to the parallel combina tion of the third and fourth resistive arms of said bridge connected in series with a primary winding of a second transformer, said series connected re sistor and transformer winding being connected across the measuring arm of said bridge;

a source of anodizing voltage having first and second,

oppositely poled terminals;

means, including a cathode electrode, connected to the first terminal of said source, for applying anodizing current from said source to said thin-film resistor to anodize said resistor and increase its resistance;

means connecting the second terminal of said source to a center-tap connection on the primary winding of said second transformer for directing the flow of anodizing current, in two equal halves, in opposite directions through the primary winding so that said thin-film resistor is uniformly and linearly anodized by equal currents flowing between said cathode electrode and opposite ends of said thin-film resistor; and

means connected to a secondary winding of said second transformer and responsive to a balanced condition Within said bridge circuit for interrupting said anodizing current when the resistance of said thinfilm resistor is equal to said preselected value.

6. A circuit, as set forth in claim 5, wherein said bridge circuit is of the balanced Kelvin bridge type.

7 7. A circuit, as set forth in claim 5, wherein said means for interrupting said anodizing current includes:

an amplifier connected to the secondary winding of said second transformer, said amplifier producing zero output voltage when said bridge is at a null condition; and

means responsive to a zero output voltage from said amplifier for interrupting the flow of anodizing current.

8. A circuit, as set forth in claim 5, wherein said means for interrupting said anodizing current includes:

a phase detector having a first input connected to the secondary winding of said second transformer and a second input connected to a secondary winding of said first transformer, said phase detector producing an output voltage when the difference in phase between the voltages at said first and second inputs is at least 180; and

means connected to the output of said phase detector and responsive to an output voltage from said phase detector for interrupting the flow of anodizingcurd rent.

- 9. A circuit, as set forth in claim 8, wherein said means connected to the output of said phase detector and responsive to an output voltage from said phase detector includes:

a normally unopera'ted relay having normally closed contacts serially connected with said source of anodizing current and a winding, said winding being energized by an output voltage from said phase detector to open the contacts and interrupt the flow of current therethrough.

References Cited UNITED STATES PATENTS 3,341,444 9/1967 Chapelle 204228 3,341.445 9/1967 Gerhard 204-228 3,365,379 1/1968 Kaiser 204228X 3,496,087 2/1970 Goodwin 204228 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

